1. Field of the Invention
This invention relates to an SHG device having a waveguide structure useful as a short wavelength light source in a high recording density optical disk system for an optical information processing system.
2. Description of the Related Art
Generally, a light source for an optical disk system is required to have at least the following four properties:
1. An output power of 2 mW or more which is sufficient for reading signals from an optical disk can be obtained.
2. An optical intensity distribution at a light emitting end having a single peak.
3. A light spot diameter, at a light emitting end, is as small as possible.
4. A wavelength as short as possible.
One known means which can potentially satisfy all of these conditions is a combination of a near-infrared light source and an SHG device with a waveguide structure made of a nonlinear optical medium. The employment of a waveguide structure allows a substantially complete phase matching between a fundamental and a second harmonic (hereinafter, abbreviated as "SH") to be realized applying a modal dispersion characteristic of a waveguide and bire-fringence of the waveguide material. The fundamental is confined in a narrow space so that a relatively high conversion efficiency of the optical frequency (wavelength) can be expected.
In order to obtain a higher conversion efficiency, however, further conditions must be satisfied such as the nonlinear optical medium having a larger nonlinear optical coefficient in relation to the frequency conversion efficiency, and phase matching between modes of the same order so that a spatial overlap between the fundamental and the SH becomes large.
In order to satisfy the condition that the optical intensity distribution at a light emitting end have a single peak, the SH must be in the lowest-order mode. Furthermore, since phase matching between modes of the same order must be realized as described above, phase matching between a lowest-order mode fundamental and a lowest-order mode SH is required.
Conventionally, LiNbO.sub.3 has been widely studied as a nonlinear optical medium for constituting a waveguide. However, the nonlinear optical coefficient d.sub.31, of LiNbO.sub.3, which concerns frequency conversion applying modal dispersion phase matching is small (d.sub.31 =6.5 pm/V), and therefore it is not possible to obtain a sufficiently large output. Moreover, since the obtained SH is green light having a wavelength of about 0.55 .mu.m (Miyazaki et al.: reports for Electro-magnetic Theory Study, BMT-78-5, 1978), the material is not sufficient from the view point of wavelength.
As a means for overcoming the above deficiencies, an SHG device with a slab (planar) waveguide has been proposed (p. 1472; IBM Technical Disclosure Bulletin, Vol. 24, No. 3, August 1981). This proposed device can accomplish phase matching between a zeroth-order mode fundamental and a zeroth-order mode SH, and production of light of a sufficiently short wavelength using KNbO.sub.3 which has larger nonlinear optical coefficients d.sub.32 (=18.3 pm/V) and d.sub.31 (=15.8 pm/V).
FIG. 1 is a schematic diagram showing the configuration of the above-mentioned conventional SHG device having a slab waveguide structure which uses a thin film of a KNbO.sub.3 single crystal. In the figure, a numeral 1 designates a substrate of crystallized quartz or the like on which a slab waveguide layer 52 made of KNbO.sub.3 is layered. Laser beam LD of a fundamental frequency is incident on one end face of the slab waveguide layer 52, and an SH emits from the other end face. An air layer which functions as a cladding layer exists on the waveguide layer 52.
The prior art literature does not mention the details of conditions for accomplishing phase matching. The inventors of the present invention designed and produced on an experimental basis an SHG device having a slab waveguide structure using a thin film of KNbO.sub.3 as shown in FIG. 1. The outline of this experiment will be described below.
At first, the outline of the design philosophy of the inventors will be described.
FIG. 4 is a graph which shows guided mode dispersion curves in which the abscissa indicates a film thickness t and the ordinate indicates an effective index N of the slab waveguide layer. In the graph, C.sub.1 indicates a zeroth-order mode dispersion curve of a fundamental, and C.sub.2 a zeroth-order mode dispersion curve of an SH. The refractive index of the substrate for one wavelength is indicated by ns and that of the slab waveguide for the wavelength by nf, the waveguide mode dispersion curves C.sub.1 and C.sub.2 rise from the points of cutoff film thickness tc (tcw, tc2w) and effective index N (nsw, ns2w), and, as the film thickness increases. The effective indices N's gradually reach nfw and nf2w, respectively. Further, it will be seen that the shorter a wavelength is, the thinner the cutoff film thickness is.
By adequately selecting a nonlinear optical medium for the waveguide layer having the refractive index nfw of the fundamental is greater than the refractive index nf2w of an SH, and a material for the substrate having the refractive index nsw of the fundamental is smaller than the refractive index ns2w of the SH, the zeroth-order mode dispersion curve C.sub.1 of the fundamental always intersects the zeroth-order mode dispersion curve C.sub.2 of the SH. When the slab waveguide layer has the film thickness t.sub.pm corresponding to the point at which the dispersion curves C.sub.1 and C.sub.2 intersect, the effective index of the fundamental is equal to that of the SH, thereby achieving phase matching between the fundamental and the SH.
Most of the materials useful for a substrate satisfy the relation of nsw&lt;ns2w. When KNbO.sub.3 is used as the material of the waveguide layer, phase matching between a zeroth-order mode fundamental and a zeroth-order mode SH is accomplished in the region where the wavelength of the fundamental is longer than 0.86 .mu.m (the wavelength of the SH is in the blue region of longer than 0.43 .mu.m). As a result, an SHG device having a slab waveguide structure which uses KNbO.sub.3 can realize phase matching between a zeroth-order mode fundamental and a zeroth-order mode SH, thereby satisfying the condition that the wavelength be sufficiently short.
FIG. 2 is a schematic diagram showing a configuration of an SHG device having a slab waveguide structure which has been manufactured on an experimental basis by the inventors. In FIG. 2, arrows a, b and c respectively indicate the crystal axes of KNbO.sub.3. A numeral 1 designates a crystallized quartz substrate cut on the {1120} plane and polished as a mirror. A slab waveguide layer 42 of a single crystal of KNbO.sub.3 having a thickness of 2.1 .mu.m and a c-axis orientation (which means that the c-axis of KNbO.sub.3 is oriented in the direction perpendicular to the substrate surface) is formed on the entire surface of the substrate 1 using an LPE (Liquid Phase Epitaxy) technique, to obtain an SHG device with a slab waveguide structure. In the figure, a numeral 4 designates a semiconductor laser device optically engaging one end face of the slab waveguide layer 42.
FIG. 3 is a graph showing the refractive indices of KNbO.sub.3, which is a nonlinear optical medium, where the abscissa indicates the wavelength (.mu.m) and the ordinate the refractive index. In the graph, na, nb and nc indicate principal refractive indices along the a-axis, b-axis and c-axis directions, respectively. When comparing the principal refractive index nb at the wavelength of 0.86 .mu.m with the principal refractive index nc at the wavelength of 0.43 .mu.m which is the half of 0.86 .mu.m, it will be found that they are substantially equal to each other. Similarly, when comparing the principal refractive index nb at the wavelength of .lambda. with the principal refractive index nc at the wavelength of .lambda./2, it will be found that the relation of nb&gt;nc is held in the region of .lambda.&gt;0.86.mu., that is, .lambda./2&gt;0.43.mu..
When setting the polarizing direction of a fundamental along the b-axis and that of an SH in the c-axis, the fundamental senses the principal refractive index nb and the SH senses the principal refractive index nc. So that in the region where the wavelength of the fundamental is longer than 0.86 .mu.m exists the relation of nfw&gt;nf2 w between the refractive index nfw of the waveguide layer for the fundamental and the refractive index nf2w of the waveguide layer for the SH. In this case, the propagation direction of the fundamental and SH is along the a-axis, and the nonlinear optical coefficient which is concerned in the frequency conversion from the fundamental to the SH is d.sub.32.
In FIG. 2, laser beam LD is at a fundamental frequency generated by the semiconductor laser device 4 having the oscillation wavelength, at room temperature of 0.88 .mu.m Laser beam LD is incident on the SHG device with the slab waveguide structure so as to propagate along the a-axis direction with respect to the slab waveguide layer 42 (10 mm in the slab waveguide layer 42 in the a-axis direction) in the TE.sub.0 mode (in which the electric field is parallel to the substrate).
The wavelength of the laser beam is adjusted in the vicinity of 0.88 .mu.m by controlling the temperature of the semiconductor laser device 4 while temperature of the SHG device is constant at room temperature. The fundamental, having the wavelength of about 0.88 .mu.m, propagate while radiately spreading in the slab waveguide layer 42. Frequency conversion occurs within the range of the propagating angle where phase matching is accomplished, and an SH is generated. The light emitted from the end face of the slab waveguide layer 42 is introduced into an infrared cut-off filter (not shown). Only light SH.sub.1 of the SH is emitted, resulting in a 0.2 mW output when an output of the semiconductor laser device 4 is 95 mW. Since it is considered that the coupling efficiency of a fundamental from the semiconductor laser device 4 to the slab waveguide layer 42 is about 30%, a calculated conversion efficiency of the slab waveguide layer 42 from a fundamental to an SH is about 1%.
The SHG device described above has a slab waveguide structure in which KNbO.sub.3 is used as a material for the waveguide satisfying two of the conditions required of a light source for an optical disk apparatus: the condition that the optical intensity distribution of the SH emission has a single peak; and the condition that the wavelength of an SH can be shortened to the blue region. As shown in FIG. 2, however, the SH at a light emitting end 42c (SH(e)) spreads in a long, and narrow shape. Consequently, a problem exists in that a lens system combining an ordinary collimator lens and focusing lens cannot condense the light to a minute spot, and that an output of 2 mW, as required for reproducing signals from an optical disk, cannot be obtained because of the insufficient confinement of a fundamental.